PROJECT SUMMARY Among homeless adults with high rates of mental health and substance use conditions, combustible tobacco use prevalence ranges from 50%?75%, nearly 3 times the rate of housed adults.1-4 Homeless adults have expressed interest in ceasing tobacco use, but face numerous barriers while homeless including physical (e.g., housing) and social (e.g., social support) environmental barriers that hinder successful cessation.1,2,5-7 Permanent supportive housing (PSH) provides opportunities to overcome these physical and social environmental barriers through housing stability and supportive services.5,8-10 However, the varying contexts of these environments, such as whether PSH residents live in clustered (congregate) or dispersed (scattered) housing models11 or receive cessation support from providers,8,12 may also lead to differential tobacco use change. Additionally, the prevalence of mental and health and substance use among this population1,13 may alter tobacco use change patterns. This study will identify whether characteristics of the social and physical environments of PSH are associated with tobacco use behavior change, and determine if mental health and substance use differentially impact tobacco use change pathways for residents. Using a concurrent nested mixed-methods design, the proposed research plan will involve secondary analysis of longitudinal quantitative and social network data from a NIDA-funded R01 study (R01DA036345; PI: Wenzel) that followed a cohort of 421 adults from homelessness to throughout their initial year in PSH. Latent growth curve modeling will be used to analyze four waves of data to determine whether physical environment (i.e., housing model, proximity to services, location, and neighborhood and building quality) and social environment (i.e., provider, structural, and functional support) characteristics of PSH are associated with longitudinal tobacco use change (Aim 1), whether social environment characteristics mediate the relationship between characteristics of the physical environment and tobacco use change (Aim 2), and whether diagnosed mental health conditions and risky substance use affect patterns of change (Aim 3). Multiple case study analysis will be used to analyze primary data from 30 qualitative semistructured interviews conducted with a purposive sample of participants from the parent study that presented with differing tobacco use change patterns (i.e., increased, decreased, maintained) to understand their perceived facilitators of and hindrances to cessation (Aim 4). The rigor of using multiple data sources (quantitative, social network, and qualitative) and integrating findings will provide a unique contribution to the literature14 and increase understanding of the effects of PSH?s environmental contexts on residents? tobacco use behavior change across their initial transition from homelessness to housing up to 3 years post housing. Findings will be used to inform new directions in cessation interventions for a high-risk, vulnerable population; specifically, how varying housing contexts, social support components, and mental health and substance use factors can be addressed to improve cessation outcomes in PSH.